1. Field of the Disclosure
The present disclosure generally relates to a fuel metering unit for a combustion engine, and more particularly, to a fuel metering unit including a variable displacement vane pump with an electronic controller for modulating the output flow thereof.
2. Description of the Related Art
Variable displacement vane pumps are known in the art, as disclosed for example in U.S. Pat. No. 5,833,438 to Sundberg. A fuel metering unit of a combustion engine that utilizes a variable displacement vane pump for precisely metering pressurized fuel to a manifold of the engine also includes associated valves and electromechanical feed back devices integrated with an electronic engine controller. The vane pump includes a rotor that turns upon operation of the metering unit, and a pivotally mounted cam ring co-axially arranged with respect to the rotor. Sliding vane elements radially extend from the rotor such that outer tips of the vane elements contact a radially inward surface of the cam ring. A cavity formed between the cam ring and the rotor includes a high pressure zone connected to an outlet of the vane pump, and a low pressure zone connected to an inlet of the vane pump. As the rotor is turned, the vane elements pump fuel from the low pressure zone to the high pressure zone. Pivoting the cam ring varies the relative positions of the rotor and the cam ring such that the amount of fuel pumped by the vane elements also varies. Controlling the position of the cam ring with respect to the rotor, therefore, controls the output of the vane pump.
One method of controlling the position of the cam ring is by using a torque motor operated servovalve. The servovalve scavenges some of the pressurized fuel exiting the vane pump and divides and directs the scavenged fuel so that a first portion of the scavenged flow is used to pivot the cam ring in a first direction, and a second portion is used to pivot the cam ring in a second direction. Altering the amounts of the first and second portions of the scavenged fuel, therefore, causes the cam ring to pivot.
The amounts of the first and second portions of the scavenged fuel produced by the servovalve is controlled by the torque motor, which is responsive to electrical signals received from an electronic controller of the turbine engine with which the fuel-metering unit is associated. U.S. Pat. No. 5,716,201 to Peck et al., for example, discloses a fuel metering unit including a vane pump, a torque motor operated servovalve and electromechanical feedback for varying the displacement of the vane pump.
It would be desirable to provide a fuel metering unit including means to provide feedback to the torque motor operated servovalve, so that the actual output of the vane pump matches a preferred output of the vane pump, as requested by the electronic engine controller. In addition, it would be desirable to provide means for damping changes in the output of the vane pump to prevent the cam ring from swinging in an uncontrolled manner.
As described in the prior art, a variable displacement vane pump also includes endplates for sealing the cavity between the rotor and the cam ring. Preferably, the endplates are tightly clamped against ends of the cam ring to prevent fuel leakage. Such tight clamping, however, makes pivotal movement of the cam ring more difficult due to the friction between the cam ring and the endplates. One solution to reducing or eliminating friction between the cam ring and the endplates while controlling fuel leakage has been to place an axial spacer radially outside of the cam ring. The axial spacer has a thickness that is slightly greater than a thickness of the cam ring, so that the endplates can be tightly clamped against the axial spacer while allowing small gaps to remain between the cam ring and the endplates to reduce or eliminate friction between the cam ring and the endplates. U.S. Pat. No. 5,738,500 to Sundberg et al., for example, discloses a variable displacement vane pump including an axial spacer.
A disadvantage of such an axial spacer, however, is that the small gaps provided between the cam ring and the endplates allow fuel leakage between the low pressure and high pressure zones formed between the cam ring and the rotor, thereby reducing pump efficiency. Therefore, it would be beneficial to provide a variable displacement vane pump that allows the cam ring to pivot without friction, while reducing fuel leakage between the low pressure and high pressure zones of the vane pump.
It is further desirable to monitor fuel flow to the engine manifold. Traditional fuel flow sensors have required electrical interfaces. Such electrical interfaces significantly increase the cost and complexity of a fuel metering system. A further undesirable characteristic of prior art fuel flow sensors is the appreciable hysteresis effect that results from side-wall friction. Thus, there is a need for a fuel flow sensor which provides control without an electrical interface. There is a further need for a fuel flow sensor without appreciable hysteresis and an accurate electromechanical sensor.
The present disclosure, accordingly, provides a fuel metering unit for a combustion engine including a servovalve having a torque motor for applying a force, a first nozzle in fluid communication with the fuel pump and a second nozzle in fluid communication with the fuel pump. An arm extends between the first and the second nozzles for varying fluid flow through the first and the second nozzles upon lateral movement of the arm. The arm is secured at a proximal end to the torque motor, whereby the arm moves upon actuation of the torque motor. A flow meter in fluid communication with an output of the fuel pump and operatively connected to a distal end of the arm variably applies a biasing force against the distal end of the arm in response to the output of the fuel pump. In another embodiment, the fuel metering unit also includes a sensor operatively associated with the flow meter for indicating a fuel flow rate output from the fuel pump.
Also disclosed is a system for indicating an output of a fuel pump including an arm for controlling the output of the fuel pump. A motor couples to a first end of the arm for positioning the arm. A housing defines an internal chamber, a primary inlet for receiving the output of the fuel pump, an outlet in fluid communication with the primary inlet, and a secondary inlet for receiving a scavenged portion of the output passing through the outlet. A valve member is slidingly received within the internal chamber such that the output and the scavenged portion exerts a force on the valve member, wherein the valve member is coupled to a second end of the arm for transmitting the force to the arm in order to assist the motor in positioning the arm. In one embodiment, the valve member is coupled to the arm by a spring.
In another embodiment, a fuel metering unit includes a variable displacement pump having a rotor including a plurality of radially extending vane slots and a cam ring coaxially arranged with respect to the rotor. The cam ring is pivotally movable between a maximum stop and a minimum stop with respect to the rotor. Vanes are slideably disposed in the radially extending vane slots for maintaining contact with the cam ring during movement thereof. A servovalve has a torque motor including an armature having opposite ends that move in opposed lateral directions in response to the torque motor receiving an electrical current from an electronic engine controller. First and second nozzles are operatively connected to an output of the variable displacement pump such that increased fluid flow through the first nozzle pivots the cam ring of the vane pump toward maximum stop while increased fluid flow through the second nozzle pivots the cam ring toward minimum stop. An elongated arm extends between the first and the second nozzles for varying fluid flow through the first and the second nozzles by movement of the elongated arm. The elongated arm is secured at a first end to the armature of the torque motor such that the elongated arm moves in response to the torque motor receiving an electrical current from the electronic engine controller. A flow meter is connected to a high pressure outlet of the vane pump and operatively connected to a second end of the elongated arm for variably applying a force against the elongated arm in response to the output of the vane pump for assisting in maintaining positioning of the elongated arm and, thereby, the cam ring.
The present disclosure also provides a vane pump including a rotor, a cam ring arranged coaxial and pivotally movable with respect to the rotor, and an axial spacer arranged coaxial with respect to the cam ring. The vane pump includes circumferential seals to reduce fuel leakage between the low pressure and high pressure zones of the vane pump in order to improve pump efficiency.
Further features of the fuel metering unit and the variable displacement vane pump according to the present disclosure will become more readily apparent to those having ordinary skill in the art to which the present disclosure relates from the following detailed description and attached drawings.